Alkali and alkaline earth metal nitrites are well known as corrosion inhibiting hydraulic cement additives for protecting steel embedded in cement compositions. Calcium nitrite in particular is a well known anodic corrosion inhibitor and is widely used in concrete for preventing corrosion of the steel reinforcing. For example, U.S. Pat. No. 3,427,175 discloses the addition of about 0.1 to 10 percent calcium nitrite to Portland cement as an accelerator and as a corrosion inhibitor. Similarly, U.S. Pat. No. 4,466,834 discloses the addition to Portland cement of stable, single phase aqueous solutions consisting essentially of water and, as solutes, a major amount by weight of calcium nitrite and a minor amount by weight of corn syrup, a hydroxycarboxylic acid, or an alkali metal or alkaline earth metal salt of hydroxycarboxylic acid. The addition of such aqueous solutions to the cement provides the corrosion inhibition of calcium nitrite without the corresponding set acceleration.
While other nitrites such as sodium nitrite can be used to inhibit corrosion, calcium nitrite is preferred inasmuch as it provides effective corrosion inhibition without many of the disadvantages encountered with other nitrites, such as reduction in compressive strength or efflorescence on brick work.
Calcium nitrite anodic corrosion inhibitors rely on the formation of a passive film on the metal surface for corrosion protection. Cathodic inhibitors are another type of inhibitor which, in a high pH environment, inhibit the cathodic reaction which accompanies the anodic dissolution of the metal. In view of the specific properties that anodic and cathodic inhibitors each provide, it would be highly desirable to obtain both anodic and cathodic inhibition in cement compositions. However, this is not a simple proposition, as addition of some cathodic inhibitors actually reduces the anodic protection of calcium nitrite.
It is therefore an object of the present invention to combine the effects of anodic and cathodic corrosion inhibition in cement compositions to achieve the benefits of each.